The present invention relates generally to radio frequency (RF) antenna systems and more particularly to omni-directional RF antenna systems responsive to energy having two polarization senses.
As is known, antenna systems for transmitting or receiving RF energy in omni-directional (i.e. 360.degree.) beam patterns have a wide variety of applications, such as in electronic countermeasures (ECM) systems. Often, it is required that such an antenna system be responsive to both energy having vertical polarization and horizontally polarized energy. One conventional antenna system comprises a vertical monopole antenna disposed to one side of a pair of crossed dipole antennas, comprising four orthogonally disposed antenna elements, the monopole and dipole antennas being supported above a ground plane conductor. Typically, a transmitter (or receiver) is coupled to an RF switch, with a first switch contact being coupled to the monopole antenna and a second switch contact being coupled through a phase shifting network to the dipole antenna elements, with each pair of dipole antenna elements being fed 180.degree. out-of-phase and in quadrature with respect to one another. The monopole antenna of such conventional antenna assembly radiates (or receives) a vertically polarized (with respect to the ground plane) beam of energy having a nominal radiation pattern which is substantially omni-directional about the monopole antenna over a relatively wide operating frequency bandwidth. The dipole antenna radiates (or receives) a horizontally polarized beam of energy with a nominal radiation pattern which is substantially omni-directional about the dipole antenna, typically over a more narrow frequency bandwidth. To extend the bandwidth of the dipole antenna, a second crossed dipole antenna having the desired operating bandwidth typically is coupled to the transmitter (or receiver) and disposed over the ground plane conductor adjacent to the monopole antenna and first dipole antenna, generally with the dipole antennas and monopole antenna being disposed side by side over the ground plane.
While the above-described dual-polarization, omni-directional antenna system has functioned satisfactorily in some applications where the adjacently disposed monopole and dipole antennas are widely spaced from each other, in other applications (such as airborne applications) requiring such antennas to be disposed closely together, the omni-directionality of the beam patterns produced thereby are severely degraded. That is, with such monopole antenna and dipole antenna or antennas disposed in close proximity with each other, the monopole antenna produces blocking or "shadowing" of the beam pattern of each of the dipole antennas in directions corresponding to the locations of the monopole antenna with respect to the dipole antennas. Likewise, the dipole antennas block or shadow the beam pattern of the monopole antenna in directions corresponding to the location of the dipole antennas with respect to the monopole antenna. The gain of each antenna is reduced (that is, signal "drop-out" is experienced) in the direction of such shadowing, resulting in a concomitant decrease in the omni-directionality of each beam pattern. Such beam pattern degradation, if sufficiently severe, produces "holes" in the coverage of the antenna system.
Accordingly, it is an object of the present invention to provide an omni-directional, dual-polarization antenna system wherein the antenna for vertically polarized energy is disposed in close proximity with the antenna for energy having horizontal polarization, with neither one of such antennas substantially adversely affecting the omni-directionality of the other one of such antennas.
It is a further object of the present invention to provide an omni-directional, dual-polarization antenna system operative over a relatively wide frequency bandwidth, such as a bandwidth of greater than one octave.